1. Field of the Invention
The present invention relates to electrically conductive sintered ceramics and to ceramic heaters using these ceramic.
2. Description of the Prior Art
Silicon carbide has an excellent hardness, strength and resistance against oxidation and corrosion. It is also known that silicon carbide has a large resistance against thermal impact under high-temperature conditions, owing to its small coefficient of thermal expansion and high heat conductivity. Silicon carbide with these excellent characteristics is a promising material for a wide field of industrial applications such as engine parts for automobiles, turbine members, heat exchanger members, crucibles for melting metals, and jigs for use at high temperatures. In order to expand the applications of silicon carbide, attempts have been made to blend silicon carbide with ceramics or intermetallic compounds to improve some characteristic of a sintered product of silicon carbide, or to impart new functions thereto. Examples of such studies are disclosed in the specifications of U.S. Pat. Nos. 3,875,476 and 3,895,219. The sintered products obtained by this prior art, however, have negative resistance-temperature coefficients and present the probability of being destroyed by heat.
Conventional glow plugs can be divided into those of the metal-sheathed type (indirect heating type) and the uncovered coil type (direct heating type). A glow plug of the sheathed type is constructed by pouring a powder of a ceramic having a resistance against heat and heat conductivity, such as magnesium oxide powder, into and around the heat-producing coil in a metal tube. With a glow plug of the uncovered coil type, on the other hand, the heat-producing coil is exposed in the engine.
A glow plug of the former type is rigid and has a great durability against the combustion gases, but is poor in heat transmission, requires a time of longer than 5 seconds before the temperature on the surface of the sheath reaches the starting point, and hence lacks a rapid heating performance. Further, if attempts are made to increase the heating speed, the temperature of the heat-producing coil increases to close to the melting point of the heat-producing element to reduce its durability considerably. Therefore an upper limit of 950.degree. C. is imposed on the surface temperature of the sheath. A glow plug of the latter type, on the other hand, can be heated rapidly, but the heat-producing element is subject to deterioration due to the combustion gases, and short-circuiting can easily develop due to the adhesion of carbon. From the standpoint of corrosion at high temperatures, furthermore, a temperature of 900.degree. C. may be the limit on the surface temperature of the heat-producing coil. Therefore, glow plugs of the sheath type are mainly used at the moment, despite their poor performance for rapid heating.
In recent years, however, an increasing demand had risen for diesel-engined automobiles to start as quickly as gasoline-engined automobiles. Under such circumstances, glow plugs of the direct heating type (surface heating type) have been reconsidered, and there is a strong desire to develop a heater element for glow plugs which can be heated quickly, and which has an excellent durability at high temperatures.